In recent years, organic thin-film transistors using organic semiconductor materials have been actively studied and developed. The organic semiconductor materials can be easily formed into a thin film by an easy process such as a wet process, for example, printing, spin coating or the like. The thin-film transistors using organic semiconductor materials also have an advantage over thin-film transistors using inorganic semiconductor materials in that the temperature of the production process can be lowered. Thus, a film can be deposited on a plastic substrate generally having a low thermal durability, so that electronic devices such as display devices can be reduced in weight and cost. Further, the electronic devices are expected to be widely used by taking advantage of flexibility of the plastic substrate.
So far, acene based materials, such as pentacene, have been reported as an organic semiconductor material of a low molecular compound (For example, Patent Literature 1 and Non-Patent Literature 1). It has been reported that an organic thin-film transistor using pentacene for an organic semiconductor layer have a relatively high field effect mobility. However, acene based materials have an extremely low solubility in a general solvent. Therefore, when such acene based material is used to form a thin organic semiconductor layer of an organic thin-film transistor, a vacuum deposition step is necessary to be performed. That is, the thin film cannot be deposited by an easy process, such as coating, printing or the like, and the acene based material does not always satisfy the expectation on the organic semiconductor material.
As one of the acene based materials similar to pentacene, 2,7-diphenyl[1]benzothieno[3,2-b][1]benzothiophene, which is a derivative of [1]benzothieno[3,2-b]benzothiophene, having a structure of the following Formula (1) (Patent Literature 2 and Non-Patent Literature 2) is deposited on a substrate treated with octadecyltrichlorosilane, so as to exhibit a mobility comparable to that of pentacene (approximately 2.0 cm2/V·s) and to have prolonged stability in an atmosphere.

Moreover, 2,7-dialkyl[1]benzothieno[3,2-b][1]benzothiophene, which is also a derivative, having a structure of the following Formula (2) (Non-Patent Literature 3) has a liquid crystal phase and high solubility, and can be applied by spin coating or casting. It is heat-treated at a relatively low temperature so as to exhibit a mobility comparable to that of pentacene (approximately 2.0 cm2/V·s).

However, 2,7-diphenyl[1]benzothieno[3,2-b][1]benzothiophene is necessary to be subjected to a vacuum deposition step similar to pentacene, and does not respond to the expectations to the organic semiconductor materials, namely, the expectation to realize an organic semiconductor material which can deposit a thin film by an easy process, such as coating, printing or the like. On the other hand, 2,7-dialkyl[1]benzothieno[3,2-b][1]benzothiophene has a relatively low transition temperature to a liquid crystal phase, approximately 100° C., and a configuration of a film formed from 2,7-dialkyl[1]benzothieno[3,2-b][1]benzothiophene may be changed by heat treatment after film deposition. Thus, there is a problem with process adaptability in production of an organic semiconductor device using such compound.
In recent years, a method of producing a field-effect transistor is reported, wherein a low molecular compound having high solvent solubility is used as a semiconductor precursor (hereinafter referred to as precursor), dissolved in a solvent and the like, and applied so as to deposit a film by a coating process, and then the film is transformed to a semiconductor, i.e., an organic semiconductor film, thereby forming a field-effect transistor. Examples of the methods include those using pentacene and similar aromatic hydrocarbon (Non-Patent Literatures 5 and 6), and those using porphyrin (for example, Non-Patent Literatures 7 and 8).
In those examples, a tetrachlorobenzene molecule detaches from a pentacene precursor, but tetrachlorobenzene has a high boiling point, and is hard to be removed from the reaction system. Additionally its toxicity is concerned.
Moreover, any of these examples has problems that semiconductor molecules which have been transformed are not stable to oxygen or water, thereby difficult to handle in an atmosphere.
From those reasons, the foregoing conventionally known compounds and precursors thereof clearly have problems with process adaptability, and a novel precursor and a method for producing the precursor have been demanded.
Patent Literature 1: Japanese Patent Application Laid-Open (JP-A) No. 05-055568
Patent Literature 2: International Publication No. WO 2006/077888
Non-Patent Literature 1: Appl. Phys. Lett. 72, p. 1854 (1998)
Non-Patent Literature 2: J. Am. Chem. Soc. 128, p. 12604 (2006)
Non-Patent Literature 3: J. Am. Chem. Soc. 129, p. 15732 (2007)
Non-Patent Literature 4: Science Vol. 270 (1995) p. 972
Non-Patent Literature 5: Optical Materials 12 (1999) p. 189
Non-Patent Literature 6: J. Appl. Phys. 79, p. 2136 (1996)
Non-Patent Literature 7: J. Appl. Phys. 100, p. 034502 (2006)
Non-Patent Literature 8: Appl. Phys. Lett. 84, 12, p. 2085 (2004)